Common ended LC filter for 100 watts inverter

[fadhel93]

Hello

I am working on a project of a 100 watts pure sine wave inverter, using DC to DC converter (push-Pull topology) as a supply for the H bridge (from 24 to 80 V ), the H bridge is controlled using mbed microcontroller. everything works fine till I connect the LC filter it draws large amount current without any load.
switching frequency=16KHz

Can anyone help me please.





schematic circuit for the H-bridge and mbed

 

[BradtheRad]

Your LC arrangement could be called a second order butterworth filter. It should be designed to resonate at a higher frequency than your switching frequency. However it is possible the two frequencies are close together.

It is essential that a load be present across the capacitor. With no load, you may expect it to draw enormous current.

A similar problem comes up with audio amplifiers, if you were to install a similar type of 2nd-order low-pass filter. Suppose you disconnect the speaker. Then play a sound which is the resonant frequency for your series LC. It will draw extreme current, with the risk of burning up components in the power amplifier.

 

[fadhel93]

thank you,
"It should be designed to resonate at a higher frequency than your switching frequency." why?

The cutoff frequency for the filter is -1/2pisqrt(LC)- about 300Hz.

If I connect ceramic capacitors rather than non-polorized Electrolytic Capacitor , it did not draw current .

 

[BradtheRad]

thank you,
"It should be designed to resonate at a higher frequency than your switching frequency." why?

Apologies for my error. I meant to say fundamental frequency.

Assuming your fundamental is 50 or 60 Hz, then you wish to filter out all higher frequencies. It is reasonable to set a rolloff frequency for 80-100 Hz.

The LC values need to be custom selected, to pass sufficient current for your load, while shaping the waveform into a sine.